thanks for the question :) I think having the some of the game played off the blockchain is fine. As long as the functionality that deals with moving funds is on the blockchain, then I think it is secure. Making a move could be done off the blockchain. You could send a move directly to the other player, and they could send back if it's a hit or not. Then the state of the game could be saved by either player pressing a save button. I'm still trying to learn this stuff :)